1. Field of the Invention
The present invention relates generally to a method for the determination of color value gradients of a picture element of a printed image when there are changes in the layer thicknesses of the inks used in the printing, whereby the picture element is photo-electrically scanned in the visible region of the spectrum, and the color value gradients are derived from the scanning signals thereby obtained.
2. Background Information
The regulation of inking in modern printing presses, in particular in offset printing, can be advantageously done on the basis of color differences. A typical regulation method based on control by color differences is described in European Patent No. B2-0 228 347 and in German Patent No. 195 15 499 C2, for example. In this method, a sheet to be printed with the printing press is divided colorimetrically into a number of test areas with regard to a selected system of color coordinates. From the color coordinates obtained in this manner, the color difference vectors are calculated to obtain the desired color coordinates with respect to said color coordinate system. These color difference vectors are converted by means of color value gradients into layer thickness change vectors, and the inking of the printing press is regulated on the basis of the layer thickness change vectors converted from the color difference vectors. The test areas used are the fields of the color test strips printed at the same time as the printed image itself.
Scanning devices have recently become available that make it possible to survey the entire image content of a printed sheet by dividing it into a large number of relatively small picture elements at reasonable cost and in a very short time, using colorimetric or spectro-photometric techniques. These scanning devices meet the theoretical instrumentation requirements because they not only use simultaneously printed test strips for the regulation of the inking of a printing press, but also acquire the color information from all the picture elements of the entire actual printed image for this purpose. One problem with this method, which is sometimes called "measurement in the image", however, results from the problem of the black fraction that is present in four-color printing which, as is known, is the result of contributions not only from the color black itself, but also from the colors superimposed on one another. A reliable determination of the color value gradients necessary for the calculation of the input variables for the color regulation for all the very different printing situations that can arise in a printed image is not possible using current methods. An additional problem results from the enormously large computer capacity required, which in practice results in unreasonably long computing times.